1. Field of the Invention
The present invention is in the field of coatings on substrates. More particularly, this invention is in the field of compositions for the deposition of coatings at high rates on glass or glass articles to provide controlled refractive index, improved emissivity characteristics, and/or appearance and abrasion resistance, and to complement or enhance other properties.
2. Description of the Prior Art
Transparent semi-conductor films such as indium oxide, cadmium stannate, or doped tin oxide, can be applied to various transparent substrates such as, e.g., soda-lime glasses, in order to reflect long-wavelength infrared radiation. Transparent dielectric films such as titanium dioxide or undoped tin oxide can be applied to transparent articles such as glass bottles to form a base coat for a second coating with a specific function. Depending on the thickness of the semiconductor or dielectric film, various reflected iridescent colors may be observed. This iridescent effect is considered to be detrimental to the appearence of the glass in applications such as windows with low emissivity, or bottles for food or beverages.
Methods and apparatus for coating glass, and especially continuous coating on moving glass, are known in the art. A description of apparatus useful in preparing a coated-glass, article is found in Lindner, U.S. Pat. No. 4,928,627, made a part of this disclosure by reference herein.
Various procedures have been devised for reducing or eliminating iridescence. For the low-emissivity application, Zaromb, in U.S. Pat. No. 3,378,396, describes an article comprising a transparent glass substrate coated with tin and silicon oxides; the coating varies gradually in composition from a high ratio of silicon oxide to tin oxide: at the substrate surface, gradually changing to almost pure tin oxide, and changing further to a ratio of not more than 60% silicon oxide to not less than 40% tin oxide at the interface of that coating with the atmosphere. The refractive index of the coating nearest to the substrate is about 1.5, substantially the refractive index of silica glass, and changes to about 2.0, the refractive index of tin oxide, at the air interface, providing an intermediate coating layer without an optical interface. The article so coated has little to no iridescence in reflected light. Zaromb teaches that aqueous solutions of tin and silicon chlorides can be spray-applied to achieve his coatings. Spray applications are usually batch operations which do not yield high-quality, uniform films; there is no mention of other means of application such as chemical-vapor deposition (CVD). He also fails to give any indication of the deposition rate, a key parameter for a commercial industrial application.
Another approach is described by Gordon in U.S. Pat. No. 4,187,336. One or more layers of a transparent material with a refractive index intermediate between that of a glass substrate and a conductive tin oxide film are deposited by atmospheric-pressure CVD between the glass and the tin oxide film. It is necessary for the intermediate layers to have specific refractive indices and thicknesses in order to be effective. It is noted that when the intermediate films contained silicon dioxide, suitable volatile compounds were found to be silane, dimethysilane, diethylsilane, tetramethyl silane, and the silicon halides. No other precursors are mentioned. The deposition rates obtained for the processes described were on the order of from 10 to 20 Angstroms per second (.ANG./sec.). Such rates are an order of magnitude below those necessary for a commercial industrial process.
In U.S. Pat. No. 4,206,252, Gordon describes a process for depositing mixed oxide and nitride coating layers of continuously varying refractive index between a glass substrate and an infra-red-reflecting coating, whereby the film iridesence is eliminated. When silicon dioxide is part of the mixed oxide film, the patent teaches that volatile silicon compounds with Si--Si and Si--H bonds are suitable precursors. Compounds such as 1,1,2,2-tetramethyldisilane, 1,1,2-trimethyldisilane, and 1,2-dimethyldisilane are disclosed. All of the compounds containing Si-Si and Si-H bonds to which reference is made are expensive, and none are comercially available.
In U.S. Pat. No. 4,386,117, Gordon describes a process for preparing mixed silicon oxide/tin oxide coatings all specific refractive indices or a continuous gradient as taught by Zaromb in U.S. Pat. No. 3,378,396, at optimum deposition rates of 80 to 125 .ANG./sec, using alkoxy-peralkylpolysilane precursors such as methoxypentamethyldisilane or dimethoxytetramethyldisilane. Again, the silica precursors cited and inferred are impractical for industrial use, because none of them is commercially available on a large scale.
Lagendijk, in U.S. Pat. No. 5,028,566, notes in column 4 that tetraethyl orthosilicate (TEOS) suffers from a number of disadvantages in its application to a substrate by low-.pressure CVD; that is, a pressure of about 500 milliTorr. These disadvantages include difficulty of doping the resultant film with phosphorus, and controlled-source delivery due to the low vapor pressure of TEOS. Lagendijk also points out that attempts at an all-liquid process to produce borophosphosilicate glass have met with limited success. He further equates the dopant effect within a broad range of phosphorus, boron, antimony, arsenic and chromium compounds, but only when used with silicon compounds having no carbon-oxygen-silicon bonds, and two or more silicon atoms.
In bottle applications, the coatings are applied at such low thicknesses, i.e., about 100 .ANG., that no iridescence is possible. However, the films are not continuous, and this discontinuity makes them unsuitable for other applications. One solution to the, discontinuity is to deposit thicker films of a material with a refractive index closer to that of the article. A mixed metal oxide/silicon oxide material deposited at a significantly more rapid rate than has heretofore been achieved would be desirable, as discussed further hereinbelow.
All the silanes disclosed in the prior art for making mixed metal oxide/silicon dioxide coatings have certain features which make them unsatisfactory for commercial development. Some are very corrosive, flammable, or oxygen-sensitive, and require special handling. Others are not readily available, or are too expensive for commercial use. Of the materials which can be used, the biggest problem which limits their commercial development in mixed metal oxide/silicon oxide and/or axynitride intermediate layers has been that of inadequate deposition rates. When the substrate is flat glass and the deposition process is CVD at ambient pressure, the deposition rate of the intermediate layers must be high enough to coat a production-line glass ribbon traveling at line speeds as high as about 15 meters per minute (m/min). Rates for deposition of the desired layers of about 350 .ANG. are desirable, and rates on the order of 400 to 600 .ANG./sec are preferable. Such rates have not heretofore been achieved under conditions which permit continuous, mass production of glass with properties.
To overcome the problems as discussed hereinabove, silica precursors are needed which are inexpensive, readily available, easy to handle, and have adequate deposition rates when vaporized with metal oxide precursors. Alkoxysilanes such as TEOS, a commodity chemical, would be desirable. However, prior to the present invention, it has not been possible to deposit silicon oxide films from TEOS by atmospheric-pressure CVD at commercially acceptable deposition rates, except at temperatures at or above 700 degrees Celsius (.degree.C.). Some success has been achieved at temperatures of from about 450.degree. to about 680.degree. C., but only by modifying the atmospheric-pressure CVD process through plasma enhancement or reduced pressure, neither of which is generally acceptable for commerical use on a continuous glass ribbon. Additives such as oxygen, ozone, or trimethyl phosphite have also been used in these modified processes, but the rates achieved are still lower than those needed for an effective commercial system.
D. S. Williams and E. A. Dein, in J. Electrochem. Soc. 134(3) 657-64 (1987), showed that phosphosilicate and borophosphosilicate glass films with controllable refractive index can be deposited at rates of about 200 .ANG./sec between 515.degree. and 680.degree. C. by the low-pressure CVD of TEOS with phosphorous or boron oxides in concentrations which varied as a function of the additive used. The low-pressure process described here is not amenable to a continuous on-line application of oxides.
In Proceedings, 2.sup.nd International ULSI Science and Technical Symposium, ECS Proceedings Vol. 98(9), 571-78 (1989), D. A. Webb et al. reported that silicon oxide films could be deposited from TEOS at rates of about 125 .ANG./sec in a plasma-enhanced CVD process using oxygen. However, plasma-enhanced CVD is not a viable option for the continuous commmercial application of oxide films to glass, being a batch process requiring complex and costly low-pressure apparatus.
A. K. Hochberg and D. L. O'Meara in J. Electrochem. Soc. 136(6) 1843 (1989) reported enhanced deposition of silicon oxide films at 570.degree. C. by CVD at low pressure when trimethylphosphite was added to TEOS. As with plasma-enhanced CVD, however, low-pressure CVD is not readily utilized for the continuous commercial application of silicon-oxide films on a moving glass sheet to produce a coated-glass article, due at least in part to the cost and complexity of the device used for deposition at low pressure.
From a review of the prior art, it cannot be determined what precursor combinations, if any, can be used for continuous deposition, under conditions and at a rate suitable for mass production, of mixed metal oxide/silicon oxide films at adequate rates from readily available and relatively inexpensive reagents.
Primary or secondary coatings on glass substrates are further useful to enhance or complement properties of either the substrate or one or more of the coatings thereon, improvement of iridesence being only one application. Other uses of coatings include, e.g., protection of the substrate surface from abrasion, addition of color to clear glass, and screening of particular wavelengths of incident radiation.